1. Field of the Invention
This invention relates to the fabrication of a quasi-solid state charge storage device capable of being repeatedly charged and discharged to yield high capacities. The storage device has one or more electrochemical cells comprised entirely of a layer of ionically conducting polymer gel electrolyte separating opposing surfaces of doped or dopable electronically conducting conjugated polymeric anode and cathode electrodes supported on lightweight porous substrates. The invention further relates to the method for producing the above charge storage devices and the method of forming electrode structures like the above of such shape, density and weight so as to be suitable for combining to form battery or capacitor devices with high reversibility, prolonged stability, and high room temperature conductivity.
2. Description of the Prior Art
Polymers with conjugated .pi.-electron backbones represent a class of polymers which can be oxidized or reduced more easily and more reversibly than conventional polymers. Charge-transfer agents referred to as dopants can favorably effect oxidation or reduction to produce conjugated polymers with conductivities approaching those acquired with metals.
Problems associated with the utilization of conducting polymers in charge storage devices include retention of processability at high conductivity levels and environmental stability. However, the advantages associated with conducting polymers, such as light weight, processability and electronic conductivity, compared to traditional metallurgical processing, suggest their potential for exploitation in commercial applications, particularly rechargeable batteries.
For successful application in batteries, optimization of cycle life and shelf life is a concern. Cycle life refers to the number of cycles of charge/discharge the battery can sustain without significant loss in performance, and is limited by factors including electrolyte degradation and polymer degradation. Shelf life refers to the retention of charge during storage over an extended period, and is limited by electrolyte impurities which can be reversibly oxidized or reduced at the polymer electrodes resulting in self-discharge of the battery. General aspects of electrically conductive polymers are discussed in further detail in Frommer, J. E. and Chance, R. R., Electrically Conductive Polymers, 5 Encycl. of Polmer Sci. and Eng'g, pgs. 462-507 (2d ed. 1986), incorporated herein by reference.
Electrical conductivity in a polymer was disclosed for polyacetylene and electrochemical doping procedures for conjugated polymers described in U.S. Pat. Nos. 4,222,903; 4,204,216; 4,321,114; 4,442,187; 4,728,589; 4,801,512; and 4,940,640 to MacDiarmid et al., incorporated herein by reference.
Other known examples of conducting polymers include polypyrrole, polythiophene and polyaniline. These conducting polymers exhibit multiple redox states so that charge can be reversibly injected and extracted from the conducting polymer. Electronic conductivity, coupled with the ability to store charge along the polymer backbone, suggests the use of conjugated polymers in charge storage devices.
Electrochemical procedures are also known which enable conjugated polymers to be electrochemically doped to a controlled degree with a wide selection of organic and inorganic ionic dopant species to either a p-type or n-type material exhibiting electrical conductivity ranging to that characteristic of metallic behavior.
Known procedures for electrochemical doping of conjugated polymers typically involve an electrochemical cell, wherein at least one of the two electrodes includes a dopable conjugated polymer as the electroactive material, and an electrolyte comprising a compound which is ionizable into one or more ionic dopant species.
P-type doping proceeds by a mechanism in which operation of the electrochemical cell effects an increase in the oxidation state of the polymer by electron transfer from the carbon atoms on the conjugatedly unsaturated polymer backbone chain, imparting a positive charge thereto and consequently attracting the dopant anions as counter ions to maintain electrical neutrality in the polymer.
N-type doping proceeds by a mechanism in which operation of the electrochemical cell effects a decrease in the oxidation state of the polymer by electron transfer to the carbon atoms on the conjugatedly unsaturated polymer backbone chain, imparting a negative charge thereto and consequently attracting the dopant cations as counter ions to maintain electrical neutrality in the polymer.
The polymer in each case becomes doped to a degree dependent upon the change effected in the oxidation state of the polymer and the dopant species concentration in the electrolyte.
Secondary batteries based on known conducting polymers, in various electrode configurations and charge states, making up one or both of the anode and cathode electrodes, and in combination with a metal material or alone, are described in the patents referenced above, as well as in U.S. Pat. Nos. 4,537,195 to Weddigen; 4,869,979 to Ohtani et al.; 4,401,545 and 4,535,039 to Naarmann et al.; 4,544,615 to Shishikura et al.; 4,804,594 to Jow et al.; 4,832,869 to Cotts; and 4,837,096 to Kimura et al., each incorporated herein by reference.
For example, a secondary battery is known which in its charged state includes a n-type cation-doped conjugated polymer as its anode-active material, and a p-type anion-doped conjugated polymer as its cathode-active material.
The discharging mechanism of the above secondary battery involves the simultaneous electrochemical undoping of the cation-doped conjugated polymer and of the anion-doped conjugated polymer, with their respective ionic dopant species being retrievably released from each of the polymers into the electrolyte system.
The charging mechanism of the above secondary battery involves the simultaneous electrochemical doping of one of the electroactive conjugated polymers (i.e., the anode-active polymer of the charged battery) with the cationic dopant species from the electrolyte, and of the other electroactive conjugated polymer (i.e., the cathode-active polymer of the charged battery) with the anionic dopant species from the electrolyte.
Practical application of conjugated polymers in commercial batteries requires the resolution of some critical development issues. For example, efficient methods are needed to process conjugated polymers into electrode structures. These methods must compensate for the limited mechanical strength associated with conjugated polymers as well as their inability to be formed by melt processing methods. Furthermore, since the rate of charge insertion and extraction in a particular conjugated polymer is dependent upon the diffusion of counterions in and out of the conjugated polymer, methods to fabricate electrodes should be amendable to thin film, high surface area construction.
In the prior art, it is known to form conjugated polymer electrodes by electropolymerization of films onto planar metal substrates. However, the inclusion of the metal substrate into an electrochemical cell reduces the effective specific energy of the cell and increases the overall weight of the cell. Also in the prior art, polymeric electrodes have been formed by pelletization of low molecular weight powders resulting from chemical polymerization. However, the electrochemical properties of these known powders are significantly less favorable than for electropolymerized films.
The use of ionically conducting polymeric electrolyte compositions in electrochemical cells is also known in the prior art, and is described in U.S. Pat. No. 4,808,496 to Hope et al. and Gray, F. M., "Solid Polymer Electrolytes," VCH, New York (1991), each incorporated herein by reference.
Hope et al. '496 describes a typical solid polymeric electrolyte composition formed by compounding a salt and a polymeric material such as polyethylene oxide. Alternatively, a solvent can be combined with the polymer to improve its film-forming properties, and with the electrolyte salt to improve diffusion into the polymer in solution.
Solid polymer electrolytes are suitable for processing into large area thin layer films and can also function as a separating layer between anode and cathode electrodes. Leakage of electrolyte from the cell is also prevented by the use of solid polymer electrolytes. Polymers containing either covalently bound ionic groups or unshared electron pairs on heteroatoms which solvate and disassociate ionic salts are known for use as the polymeric component.
Transport of ions in polymer electrolytes is coupled to relaxation of the polymer chain, with ion transport occurring only in the amorphous regions of the polymer. At room temperature, the ion-containing polymer may be extensively crystalline, with low chain mobility and concomitant reduction in conductivity. Plasticizers have been used to enhance chain mobility and increase rates of ion transport.
More recently, gel polymer electrolytes have been developed, which are two phase (solid/liquid) systems with ion conduction through the liquid phase, leading to high room temperature conductivity in the range of about 1.times.10.sup.-3 ohm.sup.-1 cm.sup.-1 (S/cm). Methods for forming various gel polymer electrolytes, composed of lithium salt-solvates of organic solvents in a polymer component, including, for example, polyacrylonitrile (PAN), poly(tetraethylene glycol diacrylate) (PEGDA), poly(vinyl pyrrolidone) (PVP), poly(vinylidene fluoride) (PVF), and poly(vinyl chloride) (PVC), are described in Watanabe et al., "Ionic Conductivity of Hybrid Films Composed of Polyacrylonitrile, Ethylene Carbonate, and LiClO.sub.4, " J. Polym. Sci. 21:939 (1983); Alamgir, M. and Abraham, K. M., "Li-Conductive Solid Polymer Electrolytes with Liquid-Like Conductivity," J. Electrochem. Soc. 137:1657 (1990); and Alamgir, M. and Abraham, K. M., "Li Ion Conductive Electrolytes Based on Poly(Vinyl Chloride)," J. Electrochem. Soc. 140:L96 (1993), each incorporated herein by reference.
Presently, the above-described polymer electrolytes have been developed predominantly for lithium battery systems with lithium metal serving as the electroactive anode material, due to the fact that many lithium salts have low lattice energies needed for dissociation in polymeric solvents. However, the use of polymer electrolytes in lithium battery systems require stability in contact with the highly reactive metal, as well as ideal lithium ion transport numbers near unity. These same electrolytes might be used in cells with conjugated polymer electrodes, without the same constraints.